Electrospray is a method of generating a very fine liquid aerosol through electrostatic charging. Electrospray, as the name implies, uses electricity in conjunction with or rather than gas to form small droplets. In electrospray, a plume of droplets is generated by electrically charging a liquid passing through a nozzle to a very high voltage. The charged liquid in the nozzle is forced to hold more and more charge until the liquid reaches a critical point at which it ruptures into a cloud of tiny, highly charged droplets.
When electrospray is used as a soft ionization method for chemical analysis, the more generally accepted term is “electrospray ionization” (ESI). Electrospray ionization is the process of generating a gas phase ion from a typically dissolved solid or liquid chemical species. This process is referred to as “soft” ionization since the molecule being ionized does not fall apart or break-up during the process.
The electrospray process has profoundly affected the field of mass spectrometry by allowing structural analysis of unlimited molecular weight, e.g., large biomolecules, and being directly compatible with liquid chromatography methods. Ionization is an important event in mass spectrometry by allowing accurate mass to charge ratio measurements of ions. A mass spectrometer is an instrument which can measure the masses and relative concentrations of atoms and molecules by evaluating a number of forces on a moving charged particle. Once an ion's mass is ascertained, this information can be used to determine its chemical composition.
While traditional electron spray ion sources have been used in the mass spectrometry of many molecules, larger than desired droplets are often generated resulting in adduct ion formation, or the bonding of molecules. Additionally, large droplets are not easily ionized, resulting in low sensitivity and signal. Moreover, many traditional electrospray ion sources are limited to producing a continuous flow of sample onto the mass spectrometer rather than a pulsed flow sample which may then be used in a time-of-flight type mass analyzer.
Throughout the drawings, identical reference numbers designate similar, but not necessarily identical, elements.